java.io

Class ObjectOutputStream

java.lang.Object

java.io.OutputStream

java.io.ObjectOutputStream

All Implemented Interfaces:

Closeable, DataOutput, Flushable, ObjectOutput, ObjectStreamConstants, AutoCloseable

public class ObjectOutputStream
extends OutputStream
implements ObjectOutput, ObjectStreamConstants

An ObjectOutputStream writes primitive data types and graphs of Java objects to an OutputStream. The objects can be read (reconstituted) using an ObjectInputStream. Persistent storage of objects can be accomplished by using a file for the stream. If the stream is a network socket stream, the objects can be reconstituted on another host or in another process.

Only objects that support the java.io.Serializable interface can be written to streams. The class of each serializable object is encoded including the class name and signature of the class, the values of the object's fields and arrays, and the closure of any other objects referenced from the initial objects.

The method writeObject is used to write an object to the stream. Any object, including Strings and arrays, is written with writeObject. Multiple objects or primitives can be written to the stream. The objects must be read back from the corresponding ObjectInputstream with the same types and in the same order as they were written.

Primitive data types can also be written to the stream using the appropriate methods from DataOutput. Strings can also be written using the writeUTF method.

The default serialization mechanism for an object writes the class of the object, the class signature, and the values of all non-transient and non-static fields. References to other objects (except in transient or static fields) cause those objects to be written also. Multiple references to a single object are encoded using a reference sharing mechanism so that graphs of objects can be restored to the same shape as when the original was written.

For example to write an object that can be read by the example in ObjectInputStream:

      FileOutputStream fos = new FileOutputStream("t.tmp");
      ObjectOutputStream oos = new ObjectOutputStream(fos);

      oos.writeInt(12345);
      oos.writeObject("Today");
      oos.writeObject(new Date());

      oos.close();
 

Classes that require special handling during the serialization and deserialization process must implement special methods with these exact signatures:

 private void readObject(java.io.ObjectInputStream stream)
     throws IOException, ClassNotFoundException;
 private void writeObject(java.io.ObjectOutputStream stream)
     throws IOException
 private void readObjectNoData()
     throws ObjectStreamException;
 

The writeObject method is responsible for writing the state of the object for its particular class so that the corresponding readObject method can restore it. The method does not need to concern itself with the state belonging to the object's superclasses or subclasses. State is saved by writing the individual fields to the ObjectOutputStream using the writeObject method or by using the methods for primitive data types supported by DataOutput.

Serialization does not write out the fields of any object that does not implement the java.io.Serializable interface. Subclasses of Objects that are not serializable can be serializable. In this case the non-serializable class must have a no-arg constructor to allow its fields to be initialized. In this case it is the responsibility of the subclass to save and restore the state of the non-serializable class. It is frequently the case that the fields of that class are accessible (public, package, or protected) or that there are get and set methods that can be used to restore the state.

Serialization of an object can be prevented by implementing writeObject and readObject methods that throw the NotSerializableException. The exception will be caught by the ObjectOutputStream and abort the serialization process.

Implementing the Externalizable interface allows the object to assume complete control over the contents and format of the object's serialized form. The methods of the Externalizable interface, writeExternal and readExternal, are called to save and restore the objects state. When implemented by a class they can write and read their own state using all of the methods of ObjectOutput and ObjectInput. It is the responsibility of the objects to handle any versioning that occurs.

Enum constants are serialized differently than ordinary serializable or externalizable objects. The serialized form of an enum constant consists solely of its name; field values of the constant are not transmitted. To serialize an enum constant, ObjectOutputStream writes the string returned by the constant's name method. Like other serializable or externalizable objects, enum constants can function as the targets of back references appearing subsequently in the serialization stream. The process by which enum constants are serialized cannot be customized; any class-specific writeObject and writeReplace methods defined by enum types are ignored during serialization. Similarly, any serialPersistentFields or serialVersionUID field declarations are also ignored--all enum types have a fixed serialVersionUID of 0L.

Primitive data, excluding serializable fields and externalizable data, is written to the ObjectOutputStream in block-data records. A block data record is composed of a header and data. The block data header consists of a marker and the number of bytes to follow the header. Consecutive primitive data writes are merged into one block-data record. The blocking factor used for a block-data record will be 1024 bytes. Each block-data record will be filled up to 1024 bytes, or be written whenever there is a termination of block-data mode. Calls to the ObjectOutputStream methods writeObject, defaultWriteObject and writeFields initially terminate any existing block-data record.

Since:

JDK1.1

See Also:

DataOutput, ObjectInputStream, Serializable, Externalizable, Object Serialization Specification, Section 2, Object Output Classes

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	ObjectOutputStream.PutField Provide programmatic access to the persistent fields to be written to ObjectOutput.

Field Summary

Fields inherited from interface java.io.ObjectStreamConstants

baseWireHandle, PROTOCOL_VERSION_1, PROTOCOL_VERSION_2, SC_BLOCK_DATA, SC_ENUM, SC_EXTERNALIZABLE, SC_SERIALIZABLE, SC_WRITE_METHOD, STREAM_MAGIC, STREAM_VERSION, SUBCLASS_IMPLEMENTATION_PERMISSION, SUBSTITUTION_PERMISSION, TC_ARRAY, TC_BASE, TC_BLOCKDATA, TC_BLOCKDATALONG, TC_CLASS, TC_CLASSDESC, TC_ENDBLOCKDATA, TC_ENUM, TC_EXCEPTION, TC_LONGSTRING, TC_MAX, TC_NULL, TC_OBJECT, TC_PROXYCLASSDESC, TC_REFERENCE, TC_RESET, TC_STRING

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	ObjectOutputStream() Provide a way for subclasses that are completely reimplementing ObjectOutputStream to not have to allocate private data just used by this implementation of ObjectOutputStream.
	ObjectOutputStream(OutputStream out) Creates an ObjectOutputStream that writes to the specified OutputStream.

Method Summary

Methods

Modifier and Type	Method and Description
protected void	annotateClass(Class<?> cl) Subclasses may implement this method to allow class data to be stored in the stream.
protected void	annotateProxyClass(Class<?> cl) Subclasses may implement this method to store custom data in the stream along with descriptors for dynamic proxy classes.
void	close() Closes the stream.
void	defaultWriteObject() Write the non-static and non-transient fields of the current class to this stream.
protected void	drain() Drain any buffered data in ObjectOutputStream.
protected boolean	enableReplaceObject(boolean enable) Enable the stream to do replacement of objects in the stream.
void	flush() Flushes the stream.
ObjectOutputStream.PutField	putFields() Retrieve the object used to buffer persistent fields to be written to the stream.
protected Object	replaceObject(Object obj) This method will allow trusted subclasses of ObjectOutputStream to substitute one object for another during serialization.
void	reset() Reset will disregard the state of any objects already written to the stream.
void	useProtocolVersion(int version) Specify stream protocol version to use when writing the stream.
void	write(byte[] buf) Writes an array of bytes.
void	write(byte[] buf, int off, int len) Writes a sub array of bytes.
void	write(int val) Writes a byte.
void	writeBoolean(boolean val) Writes a boolean.
void	writeByte(int val) Writes an 8 bit byte.
void	writeBytes(String str) Writes a String as a sequence of bytes.
void	writeChar(int val) Writes a 16 bit char.
void	writeChars(String str) Writes a String as a sequence of chars.
protected void	writeClassDescriptor(ObjectStreamClass desc) Write the specified class descriptor to the ObjectOutputStream.
void	writeDouble(double val) Writes a 64 bit double.
void	writeFields() Write the buffered fields to the stream.
void	writeFloat(float val) Writes a 32 bit float.
void	writeInt(int val) Writes a 32 bit int.
void	writeLong(long val) Writes a 64 bit long.
void	writeObject(Object obj) Write the specified object to the ObjectOutputStream.
protected void	writeObjectOverride(Object obj) Method used by subclasses to override the default writeObject method.
void	writeShort(int val) Writes a 16 bit short.
protected void	writeStreamHeader() The writeStreamHeader method is provided so subclasses can append or prepend their own header to the stream.
void	writeUnshared(Object obj) Writes an "unshared" object to the ObjectOutputStream.
void	writeUTF(String str) Primitive data write of this String in modified UTF-8 format.

Methods inherited from class java.lang.Object

clone, equals,